This invention relates to the art of fabricating pressure vessels and, more particularly, to improved methods for fabricating composite pressure vessels and to composite pressure vessels made in accordance with the improved methods.
Pressure vessels, such as hot water heaters, boilers, pressurized gas tanks and the like, have traditionally been fabricated from metal such as steel. However, in recent years, the use of composite pressure vessels has become more prevalent. These composite pressure vessels have typically been fabricated by a filament winding process which utilizes thermoset plastic resins such as epoxies, polyesters and vinylesters. Briefly, this technology is the process of impregnating dry fibers, such as fiberglass strands, with catalyzed resin prior to application to a mandrel. Preimpregnated fibers (xe2x80x9cprepregxe2x80x9d) may also be used. The mandrel and applied composite are then cured, at ambient temperature or with heat, to set-up the laminate and obtain a hard resin and fiber laminate shell. This shell is either removed from the mandrel or the mandrel itself becomes part of the finished product. Although the specific product application determines the exact function of the resin, in all cases, in all cases it serves as the support structure for keeping the continuous fiber strands in position.
The thermoset resins used in these processes can be categorized as of the low temperature commodity type which are characterized by their relative ease of use, low cost and availability. These resins have long served to meet the performance requirements of a wide range of pressure vessel products. However, these resin systems have well known drawbacks which may include their limited temperature capabilities, unsatisfactory finished product aesthetics, lack of extended durability, lack of appropriateness for recycling and manufacturing related issues such as downtime due to clean-up and material handling costs. Further, there are environmental concerns arising from worker exposure to vapor, overspray, emissions, etc. encountered during the fabrication processes. Some engineered thermoset resins improve performance through higher temperature capability, but unacceptable material costs are associated with them.
In addition, because of the materials and processes employed, composite pressure vessels prepared according to the prior art processes inherently have residual and significant internal stresses which, along with certain temperature sensitive incompatibilities of the materials, limit the pressure and temperature ranges in which the pressure vessels find use.
Thus, increasing performance demands, environmental issues, manufacturing issues and new market opportunities have emphasized the limitations of the use of thermoset resins in the manufacture of composite pressure vessels. Composite pressure vessels with higher temperature and pressure capabilities, improved appearance and greater durability and impact resistant characteristics and which, as to fabrication, are more environmentally-friendly, more cost effective and present fewer manufacturing issues, are accordingly highly desirable.
Therefore, it will be recognized by those skilled in the art that a process for fabricating composite pressure vessels which achieves improvement in all these areas requires a fundamentally different philosophy. It is to the provision of such a fundamentally improved process, and to pressure vessels made by such process that the present invention is directed and by which the following characteristics are obtained: improved contact at higher temperatures between the fiber and resin, better control over reinforcement/matrix ratio, scrap materials which can be effectively recycled, diminished regulation issues caused by emissions, higher processing speeds for the winding (or other overlaying mode) and curing steps, potential labor savings due to less material handling, floor space reduction, adaptability to automation, a safer environment for employees, simplification of processing lines and of material storage and handling, faster changeover times, faster startups, lower training costs, lower energy costs, etc. Therefore, pressure vessels fabricated according to the process are substantially stress relieved and exhibit improved performance over the prior art pressure vessels in that, inter alia, they can withstand higher pressures and temperatures, are more impact resistant and also have a significantly better finish.
It is therefore a broad object of this invention to provide an improved process for fabricating a composite pressure vessel.
It is more particular an object of this invention to provide such an improved process which enjoys advantages including, as opposed to prior art processes of fabricating composite pressure vessels: better control over reinforcement/matrix ratio, scrap materials which can be effectively recycled, diminished regulation issues caused by emissions, higher processing speeds for the winding (or alternatives to winding) and curing steps, substantial labor savings due to less material handling, floor space reduction, susceptibility to automation, a safer environment for employees, simplification of processing lines and of material storage and handling faster changeover times, faster startups, lower training costs, lower energy costs, etc.
In another aspect, it is an object of this invention to provide a process for fabricating composite pressure vessels which, in use, enjoys long term performance at least as good as that of traditional pressure vessels.
In yet another aspect, it is an object of this invention to provide high quality composite pressure vessels fabricated according to new processes.
In still yet another aspect, it is an object of this invention to provide high quality composite pressure vessels which have improved durability, impact resistance and corrosion resistance as well as higher temperature and pressure handling characteristics and which also have good machinability attributes and can therefore readily be welded, cut, drilled, threaded, stamped or the like as may be desired to produce a high quality finished product.
Briefly, these and other objects of the invention are achieved by a process of fabricating a composite vessel which includes the steps of: A) fabricating a thermoplastic liner for the vessel; B) overlaying a layer comprising fiber and a thermoplastic material (preferably by winding commingled filaments, rovings or yarns) onto the thermoplastic liner to obtain a composite intermediate structure (the fiber and thermoplastic material can be heated if desired during the overlaying, e.g. winding, step); C) heating the composite intermediate structure in a mold while applying at least one force thereto tending to urge the composite intermediate structure against and into the shape of the interior walls of the mold; D) continuing step C) until the thermoplastic liner and the overlaid layer consolidate to form a composite vessel; E) cooling the mold and composite vessel until the composite vessel is solidified; and F) removing the formed composite vessel from the mold. The at least one force applied during step C) may be obtained by introducing gas pressure into the interior of the composite intermediate structure. Suitable materials for the thermoplastic material include: polyethylene, polypropylene, polybutylene terephthalate and polyethylene terephthalate.